1. Technical Field
This invention relates generally to electrical switchgear, such as air break (or disconnect) switches used in transmission lines, and, more particularly to quick break whips for rapid arc extinguishers in such switches.
2. Background Art
Air break switches, sometimes called disconnect switches, are used in electrical transmission, or distribution systems to assist in isolating sections of line or faults. Under some conditions opening a line can be performed, reliably and without appreciable arcing of consequence, by normal opening of the switch main contacts. Under some other conditions, it is important to minimize and rapidly extinguish the arc that occurs on opening the main contacts.
A situation that raises a need for rapid arc extinction is upon a switch opening a line, typically extending for miles, where the line is connected at one end to a live source, i.e., a generator of AC power, but no load is connected to the other end of the line. The line becomes highly charged and its opening normally causes a high voltage arc. Typical system voltages are in a range of from about 69 KV. To about 230 KV. Under the described conditions, the current is of a magnitude that depends on the length of the line being disconnected from the source and is typically in a range of from about 6 to about 20 amperes; a larger current results from a greater length of disconnected line due to the capacitance between the line and ground.
The industry has recognized the desirability of rapid opening of the switch to minimize the size of the arc and to minimize restriking of the arc. Restriking of the arc occurs when the sinusoidal voltage typically at 60 Hz, rises after the current has interrupted and the contacts are still close enough for an arc that was extinguished at the zero current crossing to strike again as the voltage rises.
Ideally, the gap between switch contacts should become large enough sufficiently fast that no restrike can occur. There is, in a 60 Hz system, only about 8.3 milliseconds (ms.) between zero current crossings, or between peaks, which is a half-cycle of the AC sine wave. It is therefore preferred to get a safe separation of the contacts in less than about 8 ms. Restriking of an arc not only prolongs the time needed for switch opening, it can sometimes make it even harder to finally extinguish the arc on successive zero crossings because a restrike can cause the current and voltage to increase.
Prior known equipment has sought to have rapid contact separation to achieve arc extinction. Some air break or disconnect switches with the otherwise usual main contact and switch opening features have been provided with an arc extinction device sometimes referred to as a quick break whip or a high velocity arc interrupter. These devices are characterized by having a flexible resilient rod (or xe2x80x9cwhipxe2x80x9d) of a metal, such as beryllium-copper alloy, stainless steel or aluminum, that is conductively joined with one of the main switch contacts, and a latch or hook, of a more rigid conductor, that is conductively joined with a second main switch contact.
In operation, when the switch is initially opened, the whip slides against the latch and maintains a temporary by-pass of the main switch contacts. During that initial opening, the whip is flexed into an arcuate configuration by the latch, thus storing spring energy. After further opening, the whip is freed from the latch and its stored energy is released. The whip end then moves with high velocity away from the latch, due to the spring properties of the whip, to interrupt the circuit.
To further contribute to the speed of the whip or rod, some designs have had an arrangement with an additional spring (sometimes referred to as an accelerator spring) for storing additional contact opening energy. Some designs also have shock absorbing elements intended to keep the released whip from springing back to a location near the latch where the arc can again ignite.
The prior art equipment is commonly used but is not consistently able to avoid arc restriking and is limited in how long a line it can interrupt at a given voltage.
Other known apparatus for switching at high current and voltage includes vacuum interrupters. They can be made to perform reliable switch opening but incur a considerable disadvantage in cost compared to air switches with a quick break whip.
The present invention is directed to overcoming one or more of the problems or disadvantages associated with the prior art.
The invention is directed to such apparatus as generally described above but with a whip element of a strong and flexible nonmetallic material, such as a fiber reinforced plastic or polymer material on, or in, which there is a conductive path. Such a whip element is found to enable opening at a sufficiently fast rate to minimize any chance of arc restriking and to allow higher current, and longer lines, to be interrupted at higher voltages.
The invention is directed to such whip elements as well as to quick break arc extinguishing devices that include such a whip element and air switches that include such an arc extinguishing device.
The whip of the invention can be conveniently made from readily available fiber reinforced plastic members like, or similar to, those formerly used in fishing rods and golf club shafts. Known techniques for filament winding and resin application and curing are satisfactory for making the basic member of the whip. The required conductive path of the whip can be provided, for example, by metalizing the surface of the plastic member. It is also suitable to have sufficient conductivity within the plastic member, either by conductive, e.g., carbon, filaments or some other conductive additive. In addition, a combination of conductive filaments and a metalized surface is a further example.
By way of further example, a whip is made of a commercially available fiber reinforced plastic rod, for example of epoxy resin, on which a conductor, e.g. silver, has been plated. In addition, a conductive paint can be applied to the plastic rod to serve as the conduction path or the rod may be first painted with a conductive paint and then coated with a metal such as by plating or vacuum deposition.
The improved whip element can be used in a quick break device of otherwise known structure by replacement of the formerly present all metal rod. Improved performance is represented, for example, by one set of experiments showing whip speed under comparable conditions. An air switch with a quick break whip device having a conventional metal whip produced open gap dimensions of only about 6 to 8 inches in 8 ms. With a whip in accordance with the invention, of substantially the same size, in the same device, an open gap of between about 14 to 26 inches was attained in the same time, 8 ms. Further tests with voltage applied have shown that the increase in speed of the improved whip substantially reduces arc restrikes and can normally avoid any restriking thus reducing the size of the arc. That makes it less likely for a phase to phase fault or a phase to ground fault to occur.
The improved whip of such an example was of a carbon filament reinforced epoxy resin in a tapered, tubular configuration, substantially like a fishing rod. It has about four times the strength to weight ratio (referred to as the xe2x80x9cspecific strengthxe2x80x9d) of beryllium copper alloy, which is the strongest material commonly used in the prior art for such whips. The new whip also stands out in flexibility, i.e., the ability to bend a large amount without breaking or permanent deformation, and resilience, i.e., the ability to return to its original configuration. The very low weight of the fiber reinforced plastic, even with a metalized surface, (for example about 1.5 oz., compared to about 10 oz. for a metal whip) contributes to the high specific strength of the improved whip. Furthermore, the plastic member can be tapered and tubular for lowest weight and highest strength as is readily available from commercial sources because similar articles, with no metalizing, have been manufactured for totally unrelated purposes.
The inventive whip element can be used along with, but does not require, the prior art structures that have extra accelerator springs for additional energy storage, but which incur some extra cost and complexity. It is advantageous that the whip itself can, at least in many, if not all cases, provide all the spring energy necessary; also, shock absorbers to prevent a rebound arc are not considered necessary.
The benefits of the invention are believed attainable with no significant cost increase and obviate the need for a vacuum interrupting device in many applications.
While it is convenient to modify prior whip devices by introducing the whip element of the invention, it also can be practical and advantageous to use a whip as described in a device of a modified design which will achieve more separation of the whip tip in 8 m.s. because of the higher specific strength of the nonmetallic materials used in the whip. A further advantage is the lower force needed to bend the whip into its energy storing position thereby making it easier to operate the switch manually.
Other aspects and features of the present invention will be obtained from the entirety of the description, drawings and claims.